1. Technical Field
The present invention relates to a protective circuit of a unit cell.
2. Description of the Related Art
Lithium batteries have recently entered widespread use as a power source for industrial machines or household appliances due to advantageously high voltage, broad operating temperature range, long-term stability and high energy thereof, as compared to other cells. Such a lithium battery includes a plurality of unit cells according to optimum current and maximum current conditions. Here, an assembly of the plural unit cells is referred to as a battery pack.
The lithium battery has an allowable voltage of, for example, 3.6V to 2.0V in consideration of efficiency and safety, and a reference voltage of, for example, 2.0V is called cut-off voltage. When the battery is continuously used at a voltage lower than the cut-off voltage, internal resistance of the battery increases, causing a voltage drop. Such continuous use of the battery at voltage lower than the cut-off voltage is called over-discharge. Continued over-discharge can cause internal short circuit of unit cells, which leads to explosion of the battery. Therefore, it is necessary to avoid over-discharge of the unit cells.
FIG. 1 is a graph depicting a voltage curve when a general unit cell is connected to a load and discharged. Before connection to the load, the unit cell has an open circuit voltage (OCV) of about 3.6V. However, when the unit cell is connected to the load, the output voltage of the unit cell temporarily drops to a transition minimum voltage (TMV) as shown in Section a of FIG. 1, and then returns to a normal closed circuit voltage (CCV). Thereafter, the output voltage of the unit cell gradually drops through a discharge section, as shown in Section b. When the output voltage of the unit cell reaches the cut-off voltage, internal resistance of the cell increases, thereby causing over-discharge, as shown in Section c, in which the output voltage of the unit cell rapidly drops.
Conventionally, a protective circuit has been used in a state of being connected to the unit cell so as to cut off operation of the unit cell during the over discharge section of the unit cell. However, such a conventional protective circuit has a problem that micro-discharge continues even when the unit cell is cut off due to over-discharge, since a predetermined period of time is taken until discharge of the unit cell is completely cut off.
In addition, the conventional protective circuit has a problem that a resetting procedure is additionally required, since recovery of OCV is not recognized even though the unit cell recovers OCV from over-discharge cut-off.